Syntrophin (also known as 58kDa protein, or DAP-59) is an intracellular peripheral membrane protein expressed in many tissues. In skeletal and cardiac muscle, syntrophin's importance can be inferred from its association with dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene. Syntrophin is also complexed with other members of the dystrophin family, including utrophin and the smaller 71K and 87K dystrophin homologs. In mdx mouse skeletal and cardiac muscle, which like human DMD muscle lacks dystrophin, the association of syntrophin with the sarcolemma is severely reduced, suggesting that syntrophin may contribute significantly to the pathology of dystrophic muscle. Multiple forms of syntrophin arise from three genes and potentially from alternative splicing of their mRNAs. The goal of this proposal is to determine the function of syntrophin's interaction with dystrophin in maintaining sarcolemmal stability and the importance of syntrophin diversity. The primary focus will be on skeletal muscle, although other tissues will be used when appropriate. Immunochemical, protein chemical and cell biological experiments will be done to determine the cellular locations of syntrophin isoforms, their patterns of association with members of the dystrophin family of proteins and the composition of associated glycoprotein complexes. New syntrophin-associated proteins will also be sought. The functional domain structure of syntrophin will be studied to determine the binding sites for dystrophin and other proteins. These protein chemical and biological data will then be used to determine the effects of syntrophin dysfunction on skeletal muscle, and other tissues where appropriate. For this, a null mouse mutant lacking alpha1-syntrophin (the dystrophin-associated syntrophin isoform) will be produced by targeted gene knockout. alpha1-Syn- mice will be autopsied by a trained pathologist. Adult and embryonic alpha1-syn- muscle will be examined for total contents of the other syntrophins and dystrophin family members, for levels of sarcolemmal incorporation of these proteins, for resistance to contraction-induced lesions, and for other pathology. The surface membrane and neuromuscular and myotendinous junctions will be examined ultrastructurally for deviations from normal. The results of these studies are expected to reveal the role of syntrophins in normal muscle biology and the contribution of syntrophin dysfunction to the pathology in DMD.